1. Field of the Invention
The present invention relates to switching power supply units, and more specifically, the invention relates to switching power supply units which supply DC stabilized voltages.
2. Description of the Related Art
FIG. 9 shows a view illustrating a switching power supply unit described in Japanese Unexamined Patent Application Publication No. 11-187664. First, referring to FIG. 9, a description will be given of the main structure of a conventional switching power supply unit. A first switching circuit S1 is formed by a parallel circuit composed of a switching element Q1, a diode D1, and a capacitor C1, and a second switching circuit S2 is formed by a parallel circuit composed of a switching element Q2, a diode D2, and a capacitor C2. The second switching circuit S2, the capacitor C, and an inductor L form a series resonance circuit. Control circuits 11 and 12 are disposed in such a manner that the first and second switching elements Q1 and Q2 are alternately switched on/off between periods during which both switching elements are turned off, and a rectifying element Ds is connected parallel to a capacitor Cs to place the period of a resonance produced by the capacitor Cs.
In the switching unit having such a structure, when the first switching element Q1 is switched off, a voltage is generated at a bias winding T4 as a driving winding of the second switching element, and the second switching element Q2 is thereby turned on. Then, a control transistor is switched on in a specified time determined by a time constant circuit inside the control circuit 12 so as to turn off the second switching element Q2. In this situation, if the rectifying diode Ds on the secondary side is in a conducting state, at a timing in which the diode Ds is brought into a non-conducting state, and if the diode Ds is in a non-conducting state, at a timing in which the second switching element Q2 is turned off, that is, at a timing in which the second switching element Q2 and the rectifying diode Ds are brought into a non-conducting state, a voltage is generated at a bias winding T3 as a driving winding of the first switching element Q1 to turn on the first switching element Q1.
In this way, the first switching element Q1 and the second switching element Q2 are alternately turned on/off between the periods in which both switching elements Q1 and S2 are turned off, and, energy stored in the primary winding T1 of the transformer T during a period in which the first switching element Q1 is turned on is output as electrical energy from the secondary winding T2 during a period in which the first switching element Q1 is turned off. The output energy is rectified by the rectifying element Ds, and is smoothed by a smoothing capacitor Co. Then, a DC voltage Vo is applied to a load L1 via a detection circuit 14.
In the switching power supply unit having such a structure, a part of energy once stored in the primary winding or the inductor L during a period in which the first switching element Q1 is turned on is stored in a resonance capacitor C on the primary side when the first switching element Q1 is turned off, and then, the energy is fed back to an input voltage during a period in which the second switching element Q2 is turned on. As a result, since a circulation current circulating in the circuit exists, a ratio of the circulation current with respect to an excitation current for supplying an output current increases more as the load becomes lighter, and under no load, only the circulation current exists. When the circulation current increases, conduction losses of the transformer T, and the first and second switching elements Q1 and Q2 increase, and the efficiency of the switching power supply thereby decreases.